Ulster kept their top four bid on track with a hard-fought victory over the side who had been one rung above them on the PRO12 ladder going into last night's clash at a thoroughly miserable Ravenhill.

Ospreys proved tough, stubborn opponents and in truth Ulster were more than relieved to have pipped them at the end of what was a lacklustre performance.

Weather-wise it was one of those nights for which Ravenhill is infamous with squally rain whipped up by a swirling wind. Not ideal.

The Ospreys made light of it in the opening minutes, with Ulster on the back foot and visiting out-half Matthew Morgan zipping through with disconcerting ease to sound an early alarm. The was another when full-back Sam Davies' grubber into the in-goal area proved just a tad too heavy.

The guests' ball-retention and recycling was impressive as they encamped in the Ulster half. An excellent scrum by the Welsh yielded a richly deserved 10th minute try for open-side Sam Lewis, to which Davies added the extras. Up to that point, Ulster had barely been out of their own half.

That provoked a big response, with a powerful forward surge by the home pack. John Afoa drove to within a metre or two and the passage ended with Michael Allen appearing to have scored in the left corner. The verdict was that he had not, however.

Their dander raised by that, Ulster piled forward anew, only to infringe at the breakdown on the Ospreys' 10-metres line, allowing Ospreys to raise the brief siege. In truth it had been a poor first quarter from Ulster.

An Allen kick was good enough to see Ruan Pienaar nail the catcher and force a penalty, which he kicked comfortably from the 22 for Ulster's first points after 22 minutes.

Briefly they began to move through the gears, albeit that they were getting less than their share of the decisions from referee Nigel Owens, a fact not lost on the home crowd.

A nicely judged kick to touch by James McKinney, a mazy run Ricky Andrew and the increasingly influential role of Roger Wilson offered some cause for optimism.

But Ospreys continued to pose a real threat when they countered, with Davies underlining the point with another break on 31 minutes.

Ultimately that led to a penalty, controversially awarded against Allen following video analysis, the supposed offence being that he had used his shoulder rather than his arms in the tackle. In the eyes of the growingly disgruntled home crowd, justice was done when Davies miscued his kick.

A big Ulster scrum in the final play of the opening 40 minutes saw Ospreys cough up a penalty, but Pienaar's off-target attempt just about summed up what had been a sub-standard first half.

The introduction of Irish international Iain Henderson, who replaced Lewis Stevenson, proved Mark Anscombe's determination to kick-start his team. It worked a treat.

With little over three second-half minutes gone, Darren Cave applied the finish to his side's pressure by darting through for a try to which Pienaar added the conversion to make it 10-7.

Even so, they continued to make mistakes and their play lacked both fluency and cohesion. It was all very stop-start.

We then had the bizarre situation of Henderson scoring what looked like a try, with the stadium announcement adding affirmation. So Ulster were stunned when a second TMO verdict went against them, the ruling being that the ball had not been grounded.

Enter Nick Williams and Luke Marshall. Clearly Ulster were going for the jugular. Awarded a free in the 22 they opted to scrum. Williams picked up, Marshall drove into the tackle but two phases later they lost the ball close to the Ospreys line and had to retreat.

McKinney was withdrawn with Paul Marshall joining the battle and Pienaar moving out to 10 for the final quarter.

It was all Ulster, but they needed points to underline their superiority. Henderson was to the fore as they attempted to bank rewards.

But they had a real let-off when right-wing Jeff Hassler broke at speed into acres of space, only to be caught by Pienaar, whose intervention denied Ospreys a try. That three-point lead was looking very tenuous going into the final 10 minutes.

The rain got heavier along with the fast-tiring players' feet

Somehow the guests' centre pairing Jonathan Spratt and Ashley Beck and Aisea Natoga had enough in the tank to take play to the home line where Ulster were happy to be awarded a scrum from which they cleared the danger.

Rory Scholes came on for his Ulster debut with less than five minutes remaining. But it was the debutant's Ospreys counterpart, Eli Walker, who made a turbo-charged break only to be tackled by Pienaar, who saved the day once again.